The needs for super high speed transmission systems with a speed of more than 40 Gbit/s (for example, 100 Gbit/s) have been increasing rapidly. For this reason, development is under way for practical realization of an optical transmission system that adopts a multilevel modulation scheme applied to a radio system (for example, the QPSK modulation using four-level phase modulation). However, as the transmission-signal speed becomes higher, solving problems related to the feasibility of the electric signal circuit and problems related to the degradation of the optical transmission signal (transmission-signal spectrum degradation due to an optical filter and signal degradation due to chromatic dispersion and accumulation of optical noises) becomes more difficult.
As one of techniques for solving these problems, optical polarization multiplexing has attracted attention. A polarization multiplexed optical signal is generated by, for example, an optical signal transmitter illustrated in FIG. 1A. The optical signal transmitter has a light source (LD), a pair of modulators, and a polarization beam combiner (PBC). Continuous wave light output from the light source is spilt and guided to the pair of modulators. The pair of modulators modulate the continuous wave light respectively with corresponding data signal, and generate a pair of modulated optical signals. The polarization beam combiner generates a polarization multiplexed optical signal illustrated in FIG. 1B by combining the pair of modulated optical signals. In other words, in the polarization multiplexing, two data streams are transmitted using two polarized waves (X polarization and Y polarization) that have the same wavelength and are orthogonal to each other.
Accordingly, in the polarization multiplexing, the data speed becomes half, realizing the improvement of the characteristics of the electric-signal generation circuit and reduces the cost, size and power consumption of the circuit. In addition, the characteristics of the optical transmission system as a whole is improved, as influences from quality-degradation factors such as dispersion on the optical transmission path are reduced. Japanese Laid-open Patent Publication No. 62-24731, Japanese Laid-open Patent Publication No. 2002-344426, Japanese Laid-open Patent Publication No. 2008-172799 describe a transmission system using the polarization multiplexing. In addition, as related arts, configurations described in Japanese Laid-open Patent Publication No. 2009-63835 and Japanese Laid-open Patent Publication No. 2008-172714 have been proposed.
However, in an optical signal transmitter that generates a polarization multiplexed optical signal, a modulator is provided for each polarization as illustrated in FIG. 1A. For this reason, differences in the optical power may be generated between the polarization in the polarization multiplexed output signal, due to factors such as manufacturing variability of characteristics between the modulators (for example, loss of the LN modulator) and characteristics of the optical splitter and/or the optical combiner.